The group A streptococci produces a large number of extracellular products, some of which may play a role in the pathogenesis of streptococcal infections and their sequelae. This proposal is addressed to a study of two of these products, streptolysin O (SLO) and streptolysin S (SLS). These cytolytic toxins have been investigated for many years, but the nature of their effects on the membranes of susceptible cells, and their role in the pathogenesis of streptococcal infections is not understood. The interaction of SLO with mammalian cells will be studied with cultured cells that have been treated with certain oxygenated derivatives of cholesterol. Such cells are resistant to the effects of SLO and the correlation between toxin sensitivity and membrane cholesterol concentration will be examined. The stoichiometry of SLO-cholesterol binding will be investigated using labeled cholesterol in liposomes, and by microcalorimetry techniques. Attempts to define the membrane target of SLS will be made by studying toxin action on liposomes, whose composition will be altered to contain various membrane lipids. SLS activity will also be studied on mammalian cells after altering the phospholipid composition of the cells. The early events in the initiation of streptococcal infections will be investigated using in vitro cell cultures, and human epithelial cells as a model system. Adherence of streptococci and the mechanisms by which these organisms subsequently damage epithelial cells will be studied. The role of SLO, SLS, and other extracellular products in the latter process will be examined using mutants of group A streptococci.